Just heard a shiur from Rabbi Eytan Feiner on this topic, and he said that whichever way one holds on this issue has support from the TOrah. It is interesting that he quotes Rav Kook as saying that in the 3rd Beis Hamikdash there won’t be animal korbonos (I hope I got this right, someone please verify), as things will return to the state of Gan Eden. There were a number of gedolim who were vegetarians, and the inyan of simchas yontof and oneg shabbos is not a halachic problem.
The heter after the flood was either A) because the earth did not produce the same beneficial vegetation as before, B) because people got weaker in general and lived much less time C) The earth tilted on its axis and seasons began, causing the world to get very cold at times, and meat was beneficial for energy.
Certainly there is some mussar value in not wanting to hurt another briya for food, if avoidable. I personally am not a vegetarian, but I cannot fault one who is.